test: the playground's HTML sitemap paths per locale

- seo.htmlSitemap.paths en /sitemap, pl /pl/mapa-strony, the full-path
  rule for i18n apps; the build now holds exactly seo-sitemap-en___en,
  seo-sitemap-group-en___en, seo-sitemap-pl___pl and
  seo-sitemap-group-pl___pl (L4 WR-05 evidence)

.playground/nuxt.config.ts

@@ -10,6 +10,10 @@ export default defineNuxtConfig({
   // One post per category page, so the news category has 12 pages and the
   // smoke sees first, last and gap items.
   journal: { routes: { search: '/search' }, perPage: { category: 1 } },
+  // The HTML sitemap paths as an i18n app gives them: each locale's full
+  // path, the prefixed one included (vue-seo-layer README). The layer
+  // registers each locale's pages once, at that path, named for the locale.
+  seo: { htmlSitemap: { paths: { en: '/sitemap', pl: '/pl/mapa-strony' } } },
   i18n: {
     strategy: 'prefix_except_default',
     defaultLocale: 'en',